It has been reported that acidic peptide chains consisting of aspartic acid and/or glutamic acid molecules have high bonding affinities for hydroxyapatite, one of the component materials of the bone (1, 2). Making use of this property, techniques have been reported by which those acidic peptide chains are attached to steroid hormones (sex hormones or protein anabolic hormones, etc.), which are used for bone diseases such as osteoporosis, for endowing those steroid hormones with bone-tissue targeting ability (Japanese Patent Application Publication No. 2000-327583)(3). Further techniques have been reported by which peptide chains made of carboxylated amino acid derivatives having three or more carboxyl groups per molecule were attached to and used as bone-targeting, drug-transporting carriers for steroid hormones, methotrexate, anti-cancer antibiotics, alkylating agents or cell growth factors (Japanese Patent Application Publication No. 2002-3407)(4).
Meanwhile, there is a problem that pharmaceutical preparations of physiologically active proteins like enzymes and peptide hormones are generally made unstable when they are administered to the body, and thus undergo relatively rapid inactivation by, e.g., enzymatic degradation. For stabilizing pharmaceutical preparations of physiologically active proteins in the body, a method is known which is based on coupling the proteins to polyethylene glycol (Japanese Patent No. 2852127)(5).
Among numbers of diseases caused by congenital anomaly, there is type IV A mucopolysaccharidosis (hereinafter referred to as type A Morquio disease). In mucopolysaccharidosis, which forms a group of lysosomal diseases caused by a deficiency of enzymes necessary for the metabolism of glycosaminoglycan (hereinafter referred to as GAG), accumulation of GAG occurs in affected part of the tissue as a result of the deficiency of that enzyme. Although its clinical symptoms vary among individual patients, its common characteristic pathologies include swelling of cells caused by the accumulation of GAG in lysosomes, hypertrophy of organs, destruction of tissues and failing organs. Progressive accumulation of GAG is noted and clinical symptoms are, in majority of cases, also chronic and progressive.
Type A Morquio disease is an autosomal recessive genetic disease caused by an anomaly in the gene for a lysosomal enzyme, N-acetylgalactosamine-6-sulfate sulfatase (hereinafter referred to as GALNS) and is classified as type IV A mucopolysaccharidosis. GALNS is an enzyme that hydrolyses the sulfate groups of chondroitin-6-sulfate and keratan sulfate, which are species of GAG, and the deficiency of the enzyme causes intra-tissue deposition of GAG and its increased excretion in the urine. One of the clinical characteristics of Type A Morquio disease is bone dysplasia, and thus short statute, scoliokyphasis, brevicollis, coxa valga, and articular hyperextension have been reported to occur. Also reported are corneal opacity, deafness and cardiac valvular disorders. On the one hand, one of its characteristics that is quite different from Hunter's syndrome and Hurler's syndrome is that no psychomotor retardation is observed in patients with type A Morquio disease (6).
No effective remedy is currently available for type A Morquio disease, and bone marrow transplantation provides no more than a marginal improvement of osteopathy. Thus, most of its treatment is addressed to symptomatic therapy or control of symptoms, like orthopaedic treatment to prevent dislocation in upper cervical vertebrae. On the other hand, as main symptoms are localized in the bone and joints in type A Morquio disease, it is expected that the quality of life of the patients could be greatly improved if their osteopathy is alleviated. While substitution therapy with enzyme GALNS is contemplated for type A Morquio disease, substitution therapy using native GALNS is not expected to give any satisfactory effect considering its rapid inactivation in the body and low rate of its transfer to bone tissues including growing cartilage.